diff --git a/apps/jellyfin/docker-compose.yaml b/apps/jellyfin/docker-compose.yaml
index 1b9f345..d46d5a9 100644
--- a/apps/jellyfin/docker-compose.yaml
+++ b/apps/jellyfin/docker-compose.yaml
@@ -10,6 +10,15 @@
 # the directory exists and isn't a system path, and docker-compose
 # substitutes the value below at `docker compose up` time.
 #
+# The `${MEDIA_PATH:-/nonexistent}` default-substitution keeps
+# `validate-catalog.py` (which runs `docker compose config` without any
+# .env) from failing on the empty-string case: an empty MEDIA_PATH
+# would expand to `:/media:ro` which compose rejects as "empty section
+# between colons". /nonexistent is an obviously-wrong fallback so if it
+# ever actually reaches `docker compose up` (which requires a broken
+# install flow), the mount fails loudly instead of silently mounting
+# something random.
+#
 # TODO(image-pin): `:latest` is shaky for production — pin to a digest
 # (`jellyfin/jellyfin@sha256:...`) or a stable tag once we've verified
 # one against the upstream registry. MVP drift risk accepted.
@@ -29,7 +38,7 @@ services:
     volumes:
       - furtka_jellyfin_config:/config
       - furtka_jellyfin_cache:/cache
-      - ${MEDIA_PATH}:/media:ro
+      - ${MEDIA_PATH:-/nonexistent}:/media:ro
 
 volumes:
   furtka_jellyfin_config: